This invention, in its preferred form, relates generally to nuclear fuel rods, and more particularly, to an improved end plug-to-tube girth weld which minimizes the possibility of tube damage and allows for a simplified end plug design.
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies. Conventional designs of these fuel assemblies include top and bottom nozzles, a plurality of elongated transversely spaced guide thimbles extending longitudinally between and connected at opposite ends thereof to the nozzles, and a plurality of transverse spacer grids axially spaced along the guide thimbles. Each fuel assembly also includes a multiplicity of elongated fuel rods transversely spaced apart from one another and from the guide thimbles and supported by the grids between the top and bottom nozzles. The fuel rods each contain fissile material in the form of pellets, and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat A coolant is pumped upwardly through the core in order to extract the heat generated in the core for the production of useful work.
In the manufacture of the fuel rods, each rod is, in general sequence, capped at one end by insertion and girth welding of a bottom end plug, loaded with a plurality of nuclear fuel pellets, inserted with a spring element to prevent the pellets from shifting during transportation of the completed fuel assembly, and capped with a top end plug at the second end of the rod. The rod is normally pressurized by one of several processes with an inert gas such as nitrogen for preventing collapse of the fuel rod at beginning of life in the reactor core. The process normally includes the evacuation of air and subsequent pressurization of the rod with the inert gas and final sealing of the rod by either: (1) a girth weld of the end plug if the end plug is inserted into a prepressurized rod; or (2) a seal weld of a small channel such as an axial or radial bore which extends through the end plug, if the end plug is inserted and girth welded before pressurization.
Top end plugs of the prior art, hereinafter referred to as "two-tiered end plugs" generally are cylindrical in shape, having a first body portion with an outer diameter marginally larger than the inner diameter of the fuel rod for forced insertion therein, a second body portion with an outer diameter substantially equal to the outer diameter of the fuel rod, and a transition area comprising a shoulder between the second portion and the first portion which conforms with the end of the fuel rod in a butt joint for welding thereto. As is well known in the art, the formation of the girth weld around the circumference of the rod bonding the rod to the end plug requires that the end of the fuel rod be held in close tolerance with respect to the welding electrode while the rod and electrode are rotated one revolution with respect to each other. Thus, the welding process is best carried out with support for the rod and support for the end plug. This support of the two-tiered end plug usually takes the form of a cavity in a welding end stop, which end stop conforms to the shape of and surroundingly frictionally contacts the outer portion of the end plug during welding. This end stop prevents lateral movement of the plug and rod during welding thereof, thereby maintaining the proper distance between the rod, end plug, and the electrode.
The end stop must be a sufficient distance from the weld electrode to prevent interference with the weld and potential damage to the end stop. For this reason, two-tiered end plugs normally have substantial outer body portions axially extending from the rod end, and the girth weld is performed at the junction of the rod end and the shoulder of the outer body portion of the end plug. This weld normally results in a smooth junction between the rod end and the end plug, which is desirable for minimizing pressure drop of coolant fluid through the core, and which avoids a disjoint surface at the rod/end plug interface that could be subject to erosion or corrosion. A problem encountered with this type of girth weld is that if the electrode is biased too close to the rod end, heat will not dissipate through the rod and may cause the end of the rod to "peel back" from the weld, thereby causing rejection of the rod which requires reworking or scrapping the rod. Problems encountered with the butt joint girth weld are described in U.S. Pat. No. 3,725,635, commonly assigned to the assignee of the present invention.
An alternative weld as known in the art is performed on an end plug that lies flush with the end of the tube by performing a weld of the circumferential seam between the rod inner diameter and the plug outer diameter. However, this weld is also susceptible to causing peal back, due to the thinness of the rod wall and the location of the weld melt zone.
In addition to the problems encountered with normal welds as outlined above, two-tiered end plugs have distinct problems which may be overcome by a simplified end plug design that could be facilitated by an improved weld. The two-tiered end plug is fairly expensive to manufacture due to the high volume of material of the plug and due to the method of manufacture, which may include machining the plug from bar stock, which is wasteful of material, or molding the plug and machining it to required tolerances. Further, a problem encountered in inserting the two-tiered end plug is orienting and maintaining the end plug with the proper smaller diameter end facing the rod for insertion therein. The tendency of the end plug to become inverted could lead to severe damage of the plug, the rod, and perhaps the insertion mechanism. Also, due to the smaller diameter body portion that is inserted in the rod, the two-tiered end plug has a tendency to cock or tip, causing abrasion of the second body portion and contributing to faulty welds or improper end plug seating in the rod. Pieces of the plug may also be broken off as the skewed plug is pushed into the rod, damaging the end plug or the rod and causing foreign objects to be deposited in the fuel rod. One apparatus to overcome this problem is disclosed in U.S. Pat. No. 4,679,377, assigned to the assignee of the present invention, wherein a guide channel has a cross-sectional size smaller than that of the end plug and when contacted by the moving plug yieldably expands such that the guide channel conforms to the external surface of the end plug, thereby maintaining guiding contact therewith as the end plug is moved through the guide channel to the rod.
Therefore, what is needed is an improved weld of an end plug to a fuel rod, enabling a simplified design of the end plug that is easier and less expensive to manufacture, one that will not tip during insertion in the fuel rod, and that is less sensitive to its orientation during the insertion process.